Method of storing nitrogen trifluoride

ABSTRACT

The method of storing nitrogen trifluoride includes storing nitrogen trifluoride in a chromium-molybdenum steel vessel manufactured through a deep drawing ironing process. Nitrogen trifluoride stored in this way according to the method of this invention does not deteriorate even after two years or more have passed.

RELATED U.S. APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The present invention relates, generally, to a method of storing nitrogen trifluoride (NF₃), and more particularly, to a method of storing NF3 gas, using a chromium-molybdenum steel vessel manufactured through a deep drawing ironing (DDI) process.

BACKGROUND OF THE INVENTION

In metal sheeting, a process of manufacturing a cylindrical vessel having a closed bottom using a drawing die is referred to as a DDI process.

A machining tool comprising one pair of upper and lower pieces (punch and die) made of carbon tool steel or alloy tool steel is called the drawing die. The die is mounted to a press, and a sheet material is positioned in the press and fastened thereon. Then, when the punch presses the sheet material, the sheet material is drawn and thus shaped into the body of a cylindrical vessel in cup form such that a bottom surface and a side wall surface are integrated. The upper opening of the vessel thus produced is sealed and a valve is connected at an injection port thereof, to complete a high-pressure vessel for gas storage.

The DDI process is also referred to as plug drawing.

NF₃ gas is an expensive chemical serving as an etching agent for the fabrication of a semiconductor device. The NF₃ gas may be prepared through the direct reaction of fluorine gas and ammonia, the reaction of ammonium hydrogenfluoride (NF₄HF₂) and fluorine gas, or the electrolysis of an ammonium hydrogenfluoride melt. Commonly, NF₃ is supplied to users by being prepared as a highly pure liquid and then charged in a 20˜50 L vessel in a high-pressure gas state.

In addition, since NF₃ is used as an etching agent for the fabrication of a semiconductor device, it must have high purity of 99.99% or more.

Presently, the semiconductor process requires NF₃ gas having purity levels of 3 ppm or less N₂, 3 ppm or less O₂, 1 ppm or less CO₂, 20 ppm or less CF₄, 1 ppm or less H₂O, 1 ppm or less N₂O, and 1 ppm or less HF.

Therefore, NF₃, which is highly reactive gas, should be stored in a specific vessel so as not to be deteriorated during storage or circulation.

As a vessel for storing NF₃ gas, a manganese steel vessel has been conventionally used. The manganese steel vessel is manufactured by subjecting a pipe formed of manganese steel containing 0.5˜1.5% manganese to a shaping process using heat to produce a cylindrical sealing vessel, and then conducting polishing and washing of the vessel thus produced.

The manganese steel vessel, which is a general NF₃ storage vessel, may be used after a polishing process has been conducted on the inner surface of the vessel, to realize inner surface roughness (Ra) of 10 μm or less, and washing and drying of the vessel have been conducted in a vacuum to completely remove inner impurities.

However, even though the manganese steel vessel is processed to have an Ra of 10 μm or less without inner impurities, NF₃ stored therein becomes acidic over time, resulting in deteriorated NF₃.

Although the exact mechanism for deterioration of NF₃ has not been determined, it is inferred to result from the negative effects of impurities, that is, iron oxide (Fe_(X)O_(Y)), water, and oxygen, present in the vessel. The acidification was confirmed to be mainly caused by nitric acid.

The following reaction shows the deterioration pathway of NF₃ to nitric acid in the storage vessel. 2NF₃+Fe₂O₃+H₂O+O₂→2HNO₃+2FeF₃ 2NF₃+2FeO+H₂O+(3/2)O₂→2HNO₃+2FeF₃

In the acceptable NF₃ standard, which is flowed by NF₃ gas manufacturers and users at present, only HF acidity is determined, and acidity due to nitric acid is not determined, thus nitric acid is difficult to control.

Further, since nitric acid is detected from NF₃ stored in the manganese steel vessel, the vessel used is regarded as unsuitable for storage of NF₃.

Therefore, the development of a method of storing NF₃, which does not cause deterioration of NF₃ upon lengthy storage, is urgently required by NF₃ users and manufacturers.

BRIEF SUMMARY OF THE INVENTION

Leading to the present invention, intensive and thorough research on NF₃ storage methods resulted in the finding that NF₃ stored in a chromium-molybdenum steel vessel manufactured through a DDI process does not deteriorate even after a long time elapses, although NF₃, which is stored in a conventional manganese steel vessel having an Ra of 10 μm or less by polishing the inner surface of the vessel, is deteriorated by a reaction with impurities, including trace isolated iron and water, caused by the coarse inner texture of the conventional vessel despite such processing treatment.

Accordingly, an object of the present invention is to provide a method of storing NF₃, causing no deterioration of NF₃ even upon long storage.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method of storing NF₃ gas using a chromium-molybdenum steel vessel manufactured through a DDI process. Gas used in a semiconductor process is required to have high purity in proportion to an increase in the degree of integration of semiconductors. Thus, a vessel for storing such gas should be strictly controlled. Generally, a vessel for use in the storage of highly pure gas is subjected to a polishing process to have an Ra of 10 μm or less so as to prevent the contamination of gas charged in the vessel by water and impure particles attached to the inner surface of the vessel, and is then subjected to washing and drying processes in a vacuum to remove impurities from the inner portion thereof. However, even though the storage vessel is strictly treated as above, it suffers because NF₃ charged therein is gradually acidified.

The present inventors have observed that NF₃ gas charged in a conventional manganese steel vessel is deteriorated into impurities including NO_(X) and thus the acidity (pH) thereof increases, whereas NF₃ in a storage vessel manufactured using chromium-molybdenum steel, instead of manganese steel, through a DDI process, can be maintained in a highly pure state without increasing the acidity thereof. That is, even though a conventional manganese steel vessel is precisely processed for the inner surface thereof and completely washed and dried, NF₃ gas stored therein deteriorates over time, thus forming acidic material. After all, an acidic pH is detected from the gas. However, when NF₃ gas is charged in a chromium-molybdenum steel vessel manufactured using a DDI process, no contaminants are observed, and the gas is not deteriorated even upon storage for a long period of time.

Therefore, such a chromium-molybdenum steel vessel is found to be suitable as a storage vessel of NF₃ gas. This is because chromium-molybdenum steel is used not in pipe form as in a general manganese steel vessel but in steel sheet form, to manufacture a chromium-molybdenum steel vessel using a DDI process. In particular, a chromium-molybdenum steel vessel, which is different in vessel material and manufacturing process from a manganese steel vessel, has a uniform surface with an Ra of 5 μm or less even without additional inner treatment. Moreover, in the case where the inner surface of the chromium-molybdenum steel vessel is treated, the chromium-molybdenum steel vessel may preferably have an Ra less than 1 μm.

Even though a manganese steel vessel manufactured using a manganese steel pipe undergoes thorough inner surface treatment, impurities such as water or particles are difficult to completely remove from very fine gaps of the vessel. Further, NF₃ in the above vessel undesirably reacts with isolated iron exposed to the inner surface of the gaps of the vessel to produce iron fluoride, which is then catalyzed to accelerate the decomposition of NF₃, thus increasing the amount of N₂O or acid component.

On the other hand, in a vessel manufactured using a chromium-molybdenum steel sheet through a DDI process, the inner texture of the steel sheet becomes very dense through compression and shaping thereof, and thus the iron component is in almost the same state as if it were actually sintered. Moreover, the inner surface of the chromium-molybdenum steel vessel is uniform and clear even without the inner treatment process. This is believed to be because chromium-molybdenum steel has fewer fine spaces, such as gaps of the inner surface, than manganese steel, thus easily removing inner impurities.

Further, as the amounts of isolated iron and trace impurities are decreased, NF₃ gas is prevented from decomposition. A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention.

EXAMPLE

Ammonia and fluorine gases were supplied to an ammonium hydrogen fluoride melt to prepare impure NF₃ gas. The gas thus prepared was refined to low-temperature highly pure NF₃ in a liquid phase, which was then collected in a gaseous phase in a storage vessel. 20 kg of the collected NF₃ gas was charged in each of a manganese steel vessel and a chromium-molybdenum steel vessel manufactured through a DDI process. Each vessel was allowed to stand at room temperature, and variation in the pH of NF₃ gas was measured over time. As such, the manganese steel vessel containing 1.5 wt % of manganese and the chromium-molybdenum steel vessel containing 1.5 wt % of chromium and 0.5 wt % of molybdenum were used. N₂O was analyzed using gas chromatography (Valco, POD detector). In addition, HNO₃ was analyzed by determining total acidity through neutralization titration using NaOH, subtracting the amount of HF from the total acidity, and then converting the resultant value to the amount of HNO₃. The amount of HF was assayed using an F ion analyzer, and the presence of HNO₃ was confirmed by anion qualitative analysis using sulfuric acid and FeSO₄. The results are given in Tables 1 and 2 below, in which an Ra is a value measured on the inner surface of the vessel. TABLE 1 Acidity of Gas in Storage Vessel Storage Vessel Storage Amount (kg) pH (Color Test) Manganese Steel 20 pH 7 ---> pH 3 (47L, Ra: 25 μm) (after 2 days) Manganese Steel 20 pH 7 --> pH 5 (47L, Ra: 10 μm) (after 6 months) Chromium-Molybdenum 20 pH 7 --> pH 7 Steel (DDI) (after 2 years) *manganese steel: 1.5 wt % of manganese chromium-molybdenum steel: 1.5 wt % of chromium, and 0.5 wt % of molybdenum

TABLE 2 Gas Component after 6 Months HF HNO₃ N₂O pH Storage Vessel (ppm) (ppm) (ppm) (Color Test) Manganese Steel 0.148 3.339 1 3 (Ra: 25 μm) Manganese Steel 0.022 1.816 Trace 5 (Ra: 10 μm) Chromium-Molybdenum 0.004 0.932 No Detection 7 Steel (DDI) *manganese steel: 1.5 wt % of manganese chromium-molybdenum steel: 1.5 wt % of chromium, and 0.5 wt % of molybdenum

Although acid was detected over time in the manganese steel vessel, no variation was observed in the chromium-molybdenum steel vessel for a period of time of 2 years or longer. Further, the manganese steel vessel having an Ra of 10 μm or less caused the decomposition of gas to be much lower than the manganese steel vessel having an Ra of 25 μm or more, however decomposition was still higher than in the chromium-molybdenum steel vessel manufactured using a DDI process.

As the result of gas component analysis in Table 2, the amounts of nitric acid and fluoric acid slightly increased in the gas charged in the manganese steel vessel, and thus the color indicating pH changed. It should be noted that the amount of N₂O in the gas stored in the manganese steel vessel increased over time, whereas the gas stored in the chromium-molybdenum steel vessel subjected to DDI did not deteriorate, even after a long time, and maintained its highly pure state. Therefore, in the present invention, a vessel formed of chromium-molybdenum steel containing 1.5˜2.0 wt % of chromium and 0.2˜0.5 wt % of molybdenum is confirmed to be suitable for use in storing NF₃.

As described hereinbefore, the present invention provides a method of storing NF₃ Therefore, NF₃ gas stored in this way according to the method of the present invention does not deteriorate even after two years or more have passed.

Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

1. A method of storing nitrogen trifluoride, comprising the steps of: storing nitrogen trifluoride in a chromium-molybdenum steel vessel manufactured using a deep drawing ironing process.
 2. The method as set forth in claim 2, wherein the chromium-molybdenum steel vessel is comprised of chromium-molybdenum steel comprising 1.5˜2.0 wt % of chromium and 0.2˜0.5 wt % of molybdenum. 